Modern Rheological Measurement Methods and Anton Paar Solutions

Rheology examines the flow and deformation behavior of materials under stress. Measuring instruments used to determine rheological properties are called rheometers, and the corresponding measurement technique is called rheometry. By performing measurements with a rheometer equipped with an air-bearing motor, it is possible to characterize the production process, performance properties, application process, and behavior of the solid coating film. Rheological properties of a sample such as viscosity or elasticity can be determined with a rheometer. They can be used in process design, product development and research, and also in quality assurance. As a result of rotational tests, the dynamic viscosity η of the measured sample can be determined. The viscosity of an ideal viscous (Newtonian) fluid such as water does not change with increasing shear rate and remains constant. Most coating systems are viscoelastic materials whose viscosity changes with shear rate and whose elastic properties usually have a significant effect on application behavior. Viscoelastic materials exhibit both viscous and elastic behavior and can be examined through oscillation tests. [caption id="attachment_112052" align="aligncenter"] Figure 1: Powder and liquid paint[/caption]  

MCR (Modular Compact Rheometer) series features;

• Compact design with minimal space requirements,
• Minimization of user errors through automatic recognition and configuration of various accessories with patented ToolmasterTM technology,
• Simultaneous position control with TruStrainTM,
• Automatic gap control with TruGapTM
• Active sample temperature control with T-ReadyTM
• Air-bearing synchronized EC motor technology. Thanks to these features, MCR Rheometers are ideal instruments for rheological measurements and enable rheological measurements to be performed efficiently and with high productivity. Numerous tests can be performed with Anton Paar MCR series, ranging from routine quality control applications to research and development applications.

Thanks to the modular design that can combine special accessories, it is also possible to measure various additional parameters, for example DMTA (Dynamic Mechanical Thermal Analysis), Powder Cell, UV curing system, etc. From liquid to solid state; some of these applications in the paints and coatings industry are listed below.

Obtaining the Viscosity Curve

Viscosity measurement by rotational test is the most common way to characterize liquid samples. Since the viscosity of many samples does not follow Newton's law; in other words, since viscosity changes with shear rate, it makes sense to use a shear rate ramp to obtain the viscosity curve. [caption id="attachment_112055" align="aligncenter"] Figure 1: Viscosity curves of an ideal viscous fluid and a viscoelastic coating[/caption]   Figure 1 shows the viscosity curves of two different samples. The first is an ideal viscous fluid, such as oil; the second is a coating with shear-thinning behavior. Compared to an ideal viscous fluid, the coating shows higher viscosity at low shear rates (0.01 to 1 s-1) and is less prone to sagging on vertical surfaces. At higher shear rates, the viscosity of the coating is much lower; therefore less force will be needed to pump or apply the paint. In the shear rate range between 5 and 10 s-1, both samples show approximately the same viscosity, but the flow behavior of both samples differs significantly. It would be useful to measure the viscosity of viscoelastic samples over a wide range of shear rates.

Yield Point Determination with Flow Curves

Yield point is defined as the minimum force required for a material to start flowing. This value can be determined by a flow curve and then by fitting a regression to the curve. Depending on the yield point, various samples can be clearly distinguished from each other. Anton Paar MCR 72 and more comprehensive device configurations are ideal for such measurements. [caption id="attachment_112056" align="aligncenter"] Figure 2: "MCR x2 Rheometer" and "Applied Rheology Book"[/caption]

Time-Dependent Structural Recovery

After application (brush, roller or spray), time-dependent structural recovery is an important criterion for flow and sagging behavior. The 3-step thixotropy test (3ITT) is a good option to determine this behavior. This test can be performed in rotation or oscillation mode.

3-Step Thixotropy Test (Oscillation Test)

• Step 1 (Determining behavior before application) This interval describes the pre-application behavior of the samples. The amount of deformation applied in this measurement step is not enough to damage the material structure. The amount of deformation to be applied is selected from a previously performed deformation sweep by determining the linear viscoelastic (LVE) range (deformation range that does not damage the material).

• Step 2 (During application, destruction of structure)

In the second step, the three-dimensional force network of the sample is destroyed with high deformation or high shear stress. This section simulates the application process, for example spraying, brushing, etc.

• Step 3 (Time-dependent structural recovery after application)

In this step, the time-dependent structural recovery of the sample is determined and the measurement settings of the first step are repeated for comparison with the first step. Figure 3 shows the 3ITT measurement result for a coating. In the first step, G' (Storage Modulus, elastic part) is higher than G" (Loss Modulus, viscous part), so the sample behaves like a viscoelastic solid at rest.

Two Main Analyses Exist for This Test Result:

• Percentage of structural recovery The G' value at the end of the first step is used as a reference value (corresponding to 100% structural strength). The ratio of G' values in the first and third steps defines the percentage of structure recovery at specific and desired times.

• G' = G'' intersection point

After a certain period, the G' and G" curves intersect. This means that the material has transformed from a viscoelastic liquid to a gel-like solid after application. Until this transition point is reached, the paint can still flow and create a smooth surface. When the time to reach the intersection point is very long and is accompanied by very high layer thicknesses, "tracks," "tears," or "orange peel" effects may occur. Very rapid structural recovery can cause the paint to not have sufficient time to flow and result in surface defects remaining. Depending on coating system requirements, the required properties can be adjusted by adding additives.

DMTA (Dynamic Mechanical Thermal Analysis) Measurements of Cured Paint Films

Another way to characterize the properties of coating systems is to perform dynamic mechanical thermal analysis (DMTA) measurements of solid paint films. In DMTA measurements, the mechanical behavior of a polymer is measured over a wide temperature range. DMTA measurements and determination of the glass transition temperature Tg provide valuable information about the flexibility of the cured coating film at various temperatures. For example, the resistance of the coating can be controlled by the glass transition temperature. [caption id="attachment_112063" align="aligncenter"] Figure 3: "MCR xx2 Series Rheometer" equipped with convection heating furnace for DMTA measurement and "RheoCompass Software"[/caption] Below the glass transition temperature (Tg) a sample is hard, brittle, and glassy and is almost non-deformable. Above the glass transition temperature (Tg), the material is either in the rubber-elastic range (like a cured coating film) depending on the type of polymer, or in a melted state when the temperature increases further. For DMTA measurements, a rheometer with an air-bearing motor and a film-fiber fixture (UXF) can be used. To provide precise temperature control, a convection heating furnace with an evaporator unit and a Dewar vessel with liquid nitrogen for cooling can be mounted on the rheometer. With the UXF, a tensile test is performed, the film is clamped and stretched with a defined pre-stress to the initial temperature, and then measured with an oscillation test. Pre-stress is required to keep the sample under tension during heating. Both the pre-stress and pre-settings of the oscillation are selected so that the structure of the sample is not affected by the pre-settings. The temperature is then increased, for example at a constant heating rate of 2 K/min.

Powder Rheology: Powder Paint and Raw Material Measurements

Anton Paar offers two types of powder cells (powder flow cell and powder shear cell), providing a measurement set that allows determination of various powder properties and processing parameters. This set helps characterize and define a powder and also predict its behavior during processing, transportation, and storage. A wide variety of specialized powder measurement methods are implemented in the software, and most are quite quick. [caption id="attachment_112065" align="aligncenter"] Figure 5: Expertise areas and methods of two powder accessories for "MCR Rheometers"[/caption]   Cohesion strength measurements are one of these measurements. Cohesion strength defines the internal resistance of powder to flow and thus a measure of powder flowability. It is defined as a measure of the strength of bonding forces between powder particles. Cohesion strength measurements are quick and provide high repeatability; they help as a quality control tool to predict powder behavior. It is based on the Mohr-Coulomb theory. References 1- Anton Paar Application Note: Rheology of Coatings: Determining the Yield Point with the MCR 72 2- Anton Paar Application Note: Rheology of Coatings: Thixotropic Behavior with the MCR 72 3- Anton Paar Application Note: Modern rheological measuring methods for the coating technology 4- Anton Paar Application Note: Introduction to Powder Rheology 5-Mezger, Thomas: The Rheology-Handbook, 3rd Revised Edition, Hanover:Vincentz Network, 2011.